Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715

ENMods System: FARM3 and MiniHAM

Rev. 1.3

Page 8 of 12

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PRELIMINARY

Application Note (continued)

Off-Line Power Supply Configuration

The ENMods system maintains the DC output bus voltage

between 250 and 370Vdc over the entire input voltage range,

which is compatible with all Vicor 300V input converters.

Autoranging automatically switches to the proper bridge or

doubler mode at startup depending on the input voltage,

eliminating the possibility of damage due to improper line

connection. The ENMods system is rated at 575W output power.

These modules can serve as the AC front end for any number

and combination of compatible converters as long as the

maximum power rating is not exceeded.

Strap (ST) Pin. In addition to input and output power pin

connections, it is necessary to connect the Strap pin to the center

junction of the series hold-up capacitors (C1, C2) for proper

(autoranging) operation. Varistors V1 and V2 provide capacitor

protection. The bleeder resistors (R1, R2) discharge the hold-up

capacitors when power is switched off. Capacitors C7 and C8

are recommended if the hold-up capacitors are located more

than 3 inches from the output pins.

Enable (EN) Pin. The Enable pin must be connected to the PC or

Gate-In pin of all converter modules to disable the converters

during power-up. Otherwise, the converters would attempt to start

while the hold-up capacitors are being charged through the current

limiting thermistor, preventing the bus voltage from reaching the

thermistor bypass threshold, thus disabling the power supply. The

Enable output (the drain of an N channel MOSFET) is internally

pulled up to 15V through a 150k

Ω resistor. (see Figure 11)

A signal diode should be placed close to and in series with the

PC or (Gate-In) pin of each converter to eliminate the

possibility of control interference between converters.

The Enable pin switches to the high state (15V) with respect

to the SR pin to turn on the converters after the power-up inrush

is over. The Enable function also provides input overvoltage

protection for the converters by turning off the converters if the

DC bus voltage exceeds 400Vdc. The thermistor bypass switch

opens if this condition occurs, placing the thermistor in series

with the input voltage, which reduces the bus voltage to a safe

level while limiting input current in case the varistors conduct.

The thermistor bypass switch also opens if a fault or overload

reduces the bus voltage to less than 180Vdc. (see Figure 9)

Bus-OK (BOK) Pin. (see Figure 12) The Bus-OK pin is

intended to provide early-warning power fail information and is

also referenced to the SR pin.

Caution: There is no input to output isolation in the ENMods. It is

necessary to monitor Bus-OK via an optoisolator if it is to be

used on the secondary (output) side of the converters. A line isolation

transformer should be used when performing scope measurements.

Scope probes should never be applied simultaneously to the input and

output as this will destroy the unit.

L, N Pins. Line and neutral input.

+, – Pins. Positive and negative outputs.

SR Pin. Signal return for BOK and EN outputs

The input EMI filter consists of differential and common mode

chokes,Y– rated capacitors (line-ground) and X– rated

capacitors (line-line). This filter configuration provides

sufficient common mode and differential mode insertion loss in

the frequency range between 100kHz and 30MHz to comply

with the Level B conducted emissions limit, as illustrated in

Figures 8a thru 8c.

Hold-up Capacitors

Hold-up capacitor values should be determined according to

output bus voltage ripple, power fail hold-up time, and ride-

through time (see Figure 13). Many applications require the

power supply to maintain output regulation during a momentary

power failure of specified duration, i.e., the converters must

hold-up or ride through such an event while maintaining

undisturbed output voltage regulation. Similarly, many of these

same systems require notification of an impending power failure

in order to allow time to perform an orderly shutdown.

The energy stored on a capacitor which has been charged to

voltage V is:

(1)

Where:

C = capacitance

V = voltage across the capacitor

Energy is given up by the capacitors as they are discharged by

the converters. The energy expended (the power-time product) is:

1

2

(2)

Where:

P = operating power

∆t = discharge interval

Rearranging Equation 2 to solve for the required

capacitance:

C = 2P

∆t / (V

1

2

(3)